1. Field of the Invention
The present invention relates to a microwave antenna which, for the aiming of its beam, uses an array of elementary reflectors with active elements capable, as desired and upon activation by an optical command, of modifying the length of the path of penetration of the microwaves into the reflectors of the array to generate phase shifts varying from one elementary reflector to another and to provide for the aiming of the antenna beam.
2. Description of the Prior Art
A known antenna of this type has a reflector made out of a substrate of a dielectric material with low microwave losses, transparent to light, such as silicon dioxide SiO.sub.2 or crystallized alumina Al.sub.2 O.sub.3. On the side exposed to the microwaves, this substrate is coated with photoconductive elements that are insulated from one other by an electrically insulating material, these photoconductive elements being possibly covered with an opaque layer transparent to microwaves and arranged in on array with an lattice spacing equal to .lambda./2 to prevent multiple angles of reflection, .lambda. being the wavelength of the microwaves considered. On the opposite side, which is not exposed to the microwaves, it is coated with a electrode that is transparent to light, made of an electrically conductive material such as tin oxide.
The photoconductive elements, which may be made of "intrinsic"silicon, i.e. insulating silicon, are illuminated or not illuminated through the substrate and the transparent electrode, for example by means of a liquid crystal screen placed flat against the substrate and illuminated by a light source. When they are illuminated, they become electrically conductive and reflect the microwaves before these have penetrated the substrate. When they are not illuminated, they are electrically insulating and let the microwaves pass through them. These microwaves go through the substrate and get reflected on the transparent electrode. If the delay in propagation through the thicknesses of the photoconductive elements and of the substrate is close to an odd number of quarter periods of the microwave, the phase shift between the case where the microwaves encounter an illuminated photoconductive element and the case where they encounter a non-illuminated photoconductive element is .pi..
Thus, an array of elementary reflectors is made, with a lattice spacing equal to half the wavelength of the microwaves, each of which is capable of generating, as desired, phase shifts of 0 or .pi. upon activation by an optical command. However, if high gain of a scanning microwave antenna is to be achieved and the minor lobes and scattering are to be maintained at acceptable levels, it is generally necessary to use a controllable phase-shifter with more than two phase states at each elementary reflector.
To meet this requirement, it has been proposed to stack layers of photoconductive silicon and low loss dielectric substrate before the transparent conductive electrode to present the microwave, within each elementary reflector, with different paths of staggered lengths that correspond to various values of phase shift between 0 and 2.pi. and are a function of the depth, in the stack, of the first layer of photoconductive silicon made conductive by illumination. Difficulties then arise for the selective illumination of the different layers of photoconductive silicon which mask one another.
The present invention is aimed at overcoming these difficulties and at making it possible to obtain controllable phase-shifters with more than two phase-states in an array of reflectors for microwaves while, at the same time, preserving a simple three-layered structure for the array of reflectors, said structure being formed by a substrate made of a dielectrical material with low losses transparent to light, said substrate bearing an array of photoconductive elements on the side exposed to the microwaves and a conductive electrode transparent to light on the other side.